Oxide high-critical temperature superconductor acicular crystal and its production method

ABSTRACT

The present invention relates to a defect-free oxide high-critical temperature superconductor acicular crystal, that is, an oxide high-critical temperature superconductor acicular crystal that is substantially a perfect crystal and also relates to a method for producing the same, wherein such a crystal is essential for achieving superconducting electronic devices. The oxide high-critical temperature superconductor acicular crystal of the present invention includes an acicular crystal having a Bi 2 Sr 2 Ca 2 Cu 3 O 10  (Bi-2223) crystal structure and is grown from a powder compact by heat-treating the powder compact in an oxygen atmosphere, wherein the powder compact contains an oxide having the Bi-2223 crystal structure and TeO 2 , CaO, or (SrCa) 3 TeO 6 . The achievement of the acicular crystal having the Bi-2223 crystal structure contributes to the development of superconducting electronic devices that have been theoretically proposed but have not been achieved.

TECHNICAL FIELD

[0001] The present invention relates to a defect-free single crystal ofan oxide high-critical temperature superconductor which is essential forobtaining superconducting electronic devices, that is, an oxidehigh-critical temperature superconductor acicular crystal which issubstantially a perfect crystal and also relates to a method forproducing the same.

BACKGROUND ART

[0002] A single crystal of an oxide high-critical temperaturesuperconductor has a crystal structure in which conductive layers andnon-conductive layers are alternately stacked and the layers form anintrinsic Josephson junction. In recent years, single-crystal switchingelement devices using the intrinsic Josephson effect have been proposed.The single-crystal switching element devices, which are of a new type,are one-hundredth the size of known Josephson junction devices and havea switching speed that is 100 times higher than that of the knowndevices. The new devices are presumed to have a high operating frequencyof several THz (terahertz).

[0003] In a submicron crystal element including a Bi₂Sr₂Ca₂Cu₃O₁₀acicular crystal, superconducting single electron tunneling effect thata pair of electrons passes in the crystal has been currently observed.In order to achieve this effect, the element must operate at liquidhelium temperature (4.2 K). When crystals in which the number of layersin a unit cell is about 1000 are used, it is presumed thatsuperconductive single electron pair elements operating at liquidnitrogen temperature (77 K) can be achieved.

[0004] In order to achieve these elements, crystals having no defect orfew defect must be used. At the present, an acicular crystal of a Bioxide superconductor has the highest performance. The oxidesuperconductor has two types of crystal structure: a Bi₂Sr₂Ca₁Cu₂O₈(Bi-2212) crystal structure having a superconducting criticaltemperature of about 85 K and a Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi-2223) crystalstructure having a superconducting critical temperature of about 110 K.In research and development, acicular crystals having the Bi-2212crystal structure, of which the growth can be achieved, have been used.The inventors have succeeded in growing the acicular crystals having theBi-2212 crystal structure and extremely high crystallinity using apowder compact without performing a quenching operation and amorphizingoperation, wherein the powder compact has nominal composition of growingthe Bi-2212 acicular crystals and contains an element for reducing themelting point. The inventors have filed a patent application for thetechnique (Japanese Patent Application No. 2001-38170).

[0005] For the acicular crystals, the Bi-2223 crystal structure having asuperconducting critical temperature of about 110 K, which is muchhigher than a liquid nitrogen temperature of 77 K, is extremelyadvantageous in view of practical use as compared with the Bi-2212crystal structure having a superconducting critical temperature of about85 K. However, the acicular crystals that have been obtained only havethe Bi-2212 crystal structure and the growth of acicular crystals havingthe Bi-2223 crystal structure have not succeeded.

DISCLOSURE OF INVENTION

[0006] As described above, a method for producing an oxide high-criticaltemperature superconductor acicular crystal having no defects and theBi₂Sr₂Ca₂Cu₃O₁₀ (Bi-2223) crystal structure has not been established.The preparation of high-performance acicular crystals has not been alsoachieved.

[0007] Thus, it is a task to prepare the way for commercialization ofsuperconducting electronic devices by developing a method for producingan oxide high-critical temperature superconductor acicular crystalhaving no defects and the Bi-2223 crystal structure and then preparinghigh-performance acicular crystals. Such devices have been theoreticallyproposed but have not been brought into practical use.

[0008] In view of the above situation, it is an object of the presentinvention to provide an oxide high-critical temperature superconductoracicular crystal having few defects and the Bi-2223 crystal structure,wherein such a crystal is essential for obtaining superconductingelectronic devices. It is another object of the present invention toprovide a method for producing such a crystal.

[0009] In order to achieve the above objects, the present inventionprovides the crystals and methods below.

[0010] (1) An oxide high-critical temperature superconductor acicularcrystal includes an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure, wherein the acicular crystal is grown from a powder compactby heat-treating the powder compact at a temperature of 840 to 890° C.in an atmosphere containing 5 to 100% of oxygen, and the powder compactcontains one mol of an oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure and 0.2 to 0.8 mol of TeO₂.

[0011] (2) A method for producing an oxide high-critical temperaturesuperconductor acicular crystal includes a step of heat-treating apowder compact at 840 to 890° C. in an atmosphere containing 5 to 100%of oxygen to grow an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure, wherein the powder compact contains one mol of an oxidehaving the Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure and 0.2 to 0.8 mol of TeO₂.

[0012] (3) An oxide high-critical temperature superconductor acicularcrystal includes an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure, wherein the acicular crystal is grown from a powder compactby heat-treating the powder compact at a temperature of 840 to 890° C.in an atmosphere containing 5 to 100% of oxygen, and the powder compactcontains one mol of an oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure, 0.2 to 0.8 mol of TeO₂, and 0.1 to 2.0 mol of CaO.

[0013] (4) A method for producing an oxide high-critical temperaturesuperconductor acicular crystal includes a step of heat-treating apowder compact at 840 to 890° C. in an atmosphere containing 5 to 100%of oxygen to grow an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure, wherein the powder compact contains one mol of an oxidehaving the Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, 0.2 to 0.8 mol of TeO₂,and 0.1 to 2.0 mol of CaO.

[0014] (5) An oxide high-critical temperature superconductor acicularcrystal includes an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure, wherein the acicular crystal is grown from a powder compactby heat-treating the powder compact at a temperature of 840 to 890° C.in an atmosphere containing 5 to 100% of oxygen, and the powder compactcontains one mol of an oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure and 0.2 to 0.8 mol of an oxide having a (SrCa)₃TeO₆ crystalstructure.

[0015] (6) A method for producing an oxide high-critical temperaturesuperconductor acicular crystal includes a step of heat-treating apowder compact at 840 to 890° C. in an atmosphere containing 5 to 100%of oxygen to grow an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure, wherein the powder compact contains one mol of an oxidehaving the Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure and 0.2 to 0.8 mol of anoxide having a (SrCa)₃TeO₆ crystal structure.

BEST MODE FOR CARRYING OUT THE INVENTION

[0016] In the present invention, the preparation of an acicular crystalhas succeeded according to the following procedure: a powdery oxidehigh-critical temperature superconductor having a Bi₂Sr₂Ca₂Cu₃O₁₀(Bi-2223) crystal structure is mixed with a powder containing TeO₂, CaO,and the like; the mixture is formed into powder compacts; and the powdercompacts are then heat-treated in an atmosphere in which the oxygenpartial pressure is varied, thereby directly preparing the acicularcrystal having the Bi-2223 crystal structure using the compact.

[0017] Known acicular crystals have been grown from a calcinedmultiphase powder. Therefore, it has not been able to grow the acicularcrystal having the Bi-2223 crystal structure.

[0018] In the present invention, the acicular crystal having the Bi-2223crystal structure is directly grown from the compacts prepared accordingto the following procedure: a single phase powder having the Bi-2223crystal structure is prepared in advance by a special method; the singlephase powder is mixed with powdery TeO₂, powdery TeO₂ and CaO, orpowdery (SrCa)₃TeO₆, which enable the acicular crystal to grow; and themixture is formed into the powder compacts. This production method andthe acicular crystal grown thereby are completely new, and the acicularcrystal having a superconducting critical temperature of 110 K can beobtained.

[0019] Embodiments of the present invention will now be described indetail.

[0020] (1) Effect of single phase Bi-2223 crystal structureInvestigation has been conducted on the growth of acicular crystalshaving a Bi-2201 crystal structure, Bi-2212 crystal structure, orBi-2223 crystal structure, wherein the Bi-2201 crystal structure,Bi-2212 crystal structure, and Bi-2223 crystal structure have asuperconducting critical temperature of 20 K or less, about 85 K, andabout 110 K, respectively. During that process, the inventors found thatthe crystal structure of the acicular crystals depends on the crystalstructure of powder compacts, namely, host phases. In order to obtain adefect-free, single-phase acicular crystal having the Bi-2223 crystalstructure, the acicular crystal must be grown from a single-phase powdercompact, namely, a host phase, having the Bi-2223 crystal structure.

[0021] (2) Effects of addition of TeO₂ and addition of both TeO₂ and CaO

[0022] The growth of acicular crystals is promoted in proportion to adifference between the melting point of an oxide high-criticaltemperature superconductor and that of a host phase having nominalcomposition. Therefore, it is extremely effective that the compositioncontains TeO₂, which reduces the melting point of the host phase. Anacicular crystal having the Bi-2223 crystal structure is grown from amixture containing one mol of the oxide Bi₂Sr₂Ca₂Cu₃O₁₀ and 0.2-0.8 molof TeO₂. When the TeO₂ content is about 0.5 mol, the optimum effect canbe achieved. The obtained acicular crystal does not contain Te.

[0023] When the host phase contains both TeO₂ and CaO, the host phasehas low melting point, thereby promoting the growth of the acicularcrystal. When the host phase contains TeO₂ only or both TeO₂ and CaO,the host phase has a (SrCa)₃TeO₆ crystal structure.

[0024] (3) Effect of addition of (SrCa)₃TeO₆

[0025] In order to grow the acicular crystal having the Bi-2223 crystalstructure, the diffusional driving force for growing the acicularcrystal in the host phase is necessary. (SrCa)₃TeO₆ contained in thehost phase has the driving force. The acicular crystal is grown in apowder compact containing one mol of an oxide having a Bi₂Sr₂Ca₂Cu₃O₁₀crystal structure and 0.2-0.8 mol of an oxide having a (SrCa)₃TeO₆crystal structure. When the (SrCa)₃TeO₆ content is about 0.5 mol, theoptimum effect can be achieved.

[0026] (4) Effect of temperature and atmosphere of heat treatment

[0027] In order to grow acicular crystals, the temperature andatmosphere of heat treatment must be optimized. The acicular crystalhaving the Bi-2223 crystal structure is grown at 840-890° C. in anatmosphere containing 5-100% of oxygen during the heat treatment. Theoptimum conditions are as follows: the heat-treating temperature is 860°C. and the oxygen content in the atmosphere is 10%.

EXAMPLES

[0028] (1) Effect of single phase Bi-2223 crystal structure

[0029] A powder with nominal composition for Bi-2223 was prepared by achemical coprecipitation method. A powder compact prepared using thepowder was heat-treated at 845-850° C. for 100 hours in an atmospherecontaining 20% of O₂, thereby obtaining a single phase pellet having aBi-2223 crystal structure. The pellet was crashed in anhydrous alcoholwith a ball mill so as to avoid the hydrolysis of the pellet, therebypreparing a single phase powder having the Bi-2223 crystal structure. Inorder to render the Bi-2223 crystal structure to be a single phase, thepowder must have nominal composition represented by (BiPb)₂Sr₂Ca₂Cu₃O₁₀in which part of Bi is replaced with Pb.

[0030] As is generally known, the nominal composition is represented byBi_(1.6-1.8)Pb_(0.3-0.4)Sr_(1.9)Ca_(2.1)Cu_(3.0)O_(x) in particular

[0031] On the other hand, a calcined multiphase powder withsubstantially the same nominal composition as the above was prepared,wherein the powder had the Bi-2212 crystal structure containing Ca₂CuO₃,Ca₂PbO₄, and the like. These powders were mixed with TeO₂ and CaO insuch a manner that the ratio of an oxide having a Bi₂Sr₂Ca₂Cu₃O₁₀crystal structure to TeO₂ to CaO is 1:0.5:1.0 on a mole basis, themixtures were calcined at 820° C. for 10 hours, and the resultingmixtures were then formed into powder compacts having a diameter φ of 15mm and a thickness of 2 mm.

[0032] The powder compacts were heat-treated at 860°0 C. for 100 hoursin an atmosphere containing 10% of oxygen, thereby growing acicularcrystals from the respective powder compacts. Table 1 shows the crystalstructure of the powder compacts, namely, host phases, and the crystalstructure of the acicular crystals. TABLE 1 Crystal Structure of CrystalStructure of Host Phase Acicular Crystal Calcined Powder HavingBi-2212 + Ca₂CuO₃ + Bi-2212 Nominal Composition for Ca₂PbO₄, and thelike Forming Bi-2223 Crystal (TeO₂ + CaO) Structure Powder HavingBi-2223 Bi-2223 (TeO₂ + CaO) Bi-2223 Crystal Structure

[0033] The acicular crystal having the Bi-2212 crystal structure isgrown from the host phase having the Bi-2212 crystal structure, and theacicular crystal having the Bi-2223 crystal structure is grown from thehost phase having the Bi-2223 crystal structure. That is, the crystalstructure of an acicular crystal depends on the crystal structure of asuperconductor in a host phase. The single-phase acicular crystal havingthe Bi-2223 crystal structure can be grown only from the powder compactin which the host phase has the Bi-2223 crystal structure.

[0034] (2) Effects of addition of TeO₂ and addition of both TeO₂ and CaO

[0035] An oxide having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure was mixedwith TeO₂ or both TeO₂ and CaO, thereby preparing mixed powders havingdifferent compositions. The powders were calcined at 820° C. for 10hours and then formed into powder compacts having a diameter φ of 15 mmand a thickness of 2 mm. The powder compacts were heat-treated at 860°C. for 100 hours in an atmosphere containing 10% of oxygen, therebygrowing acicular crystals from the respective powder compacts. Table 2shows the length of the acicular crystals, obtained by varying thecontent of TeO₂ or the content of both TeO₂ and CaO, having the Bi-2223crystal structure. TABLE 2 Length of Acicular TeO₂ (mol) CaO (mol)Crystal (mm) 0 0 0 0.2 0 1˜2 0.5 0 6˜8 0.8 0 1˜3 0.5 0.1 6˜9 0.5 0.57˜10 0.5 1.0 9˜12 0.5 1.5 8˜10 0.5 2.0 3˜5

[0036] The acicular crystals having the Bi-2223 crystal structure aregrown from the powder compacts containing one mol of the oxideBi₂Sr₂Ca₂Cu₃O₁₀ and 0.2-0.8 mol of TeO₂. When the TeO₂ content is about0.5 mol, the optimum effect can be achieved and the crystal length is6-8 mm. For a combination of TeO₂ and CaO, the acicular crystals aregrown from the powder compacts having a CaO content of 0.1-2.0 mol whenthe TeO₂ content is 0.5 mol which is the most effective value. When theCaO content is about 1.0 mol, the optimum effect can be achieved and thecrystal length is 9-12 mm. The growth of the acicular crystals ispromoted due to the addition of both TeO₂ and CaO.

[0037] For the powder compacts containing no Te, the growth of theacicular crystals is not observed. The obtained acicular crystals do notcontain Te.

[0038] For both the powder compacts containing TeO₂ and the powdercompacts containing both TeO₂ and CaO, the host phases have the(SrCa)₃TeO₆ crystal structure.

[0039] (3) Effect of addition of (SrCa)₃TeO₆

[0040] An oxide having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure was mixedwith (SrCa)₃TeO₆ thereby preparing mixed powders having differentcompositions. The powders were calcined at 820° C. for 10 hours and thenformed into powder compacts having a diameter φ of 15 mm and a thicknessof 2 mm. The powder compacts were heat-treated at 870° C. for 100 hoursin an atmosphere containing 10% of oxygen, thereby growing acicularcrystals from the respective powder compacts. Table 3 shows the lengthof the acicular crystals, obtained by varying the content of(SrCa)₃TeO₆, having the Bi-2223 crystal structure. TABLE 3 (SrCa)₃TeO₆(mol) Length of Acicular Crystal (mm) 0.2 1˜2 0.5 5˜7 0.8 1˜2

[0041] The acicular crystals having the Bi-2223 crystal structure aregrown from the powder compacts containing one mol of the oxideBi₂Sr₂Ca₂Cu₃O₁₀ and 0.2-0.8 mol of (SrCa)₃TeO₆. When the (SrCa)₃TeO₆content is about 0.5 mol, the optimum effect can be achieved and thecrystal length is 5-7 mm. The obtained acicular crystals do not containTe.

[0042] (4) Effect of temperature and atmosphere of heat treatment

[0043] One mol of an oxide having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structurewas mixed with 0.5 mol of TeO₂ and 1.0 mol of CaO, thereby preparing amixed powder. The powder was calcined at 820° C. for 10 hours and thenformed into powder compacts having a diameter φ of 15 mm and a thicknessof 2 mm. The powder compacts were heat-treated at different temperaturesfor 100 hours in atmospheres having different oxygen percentages, herebygrowing acicular crystals from the respective powder compacts. Theoxygen content was controlled by adding argon. Table 4 shows the lengthof the acicular crystals, obtained by varying the heat-treatingtemperature and oxygen content, having the Bi-2223 crystal structure.TABLE 4 Heat Treating Oxygen Length of Acicular Temperature (° C.)Content (%) Crystal (mm) 840 10 1˜2 860 10 9˜12 880 10 6˜8 890 10 2˜3860 5 4˜6 860 20 7˜9 860 100 2˜4

[0044] The acicular crystals having the Bi-2223 crystal structure aregrown from the powder compacts heat-treated at 840-890° C., in anatmosphere containing 10% of oxygen. When th temperature is 860° C., thecrystal length is 9-12 mm. The acicular crystals are grown from thepowder compacts treated in an atmosphere containing 5 to 100% of oxygenwhen the heat-treating temperature is 860° C. which is most effective inthe growth. When the oxygen content is 10%, the crystal length is 9-12mm. Thus, the optimum conditions of the heat treatment are as follows:the temperature is 860° C. and the oxygen content in the atmosphere is10%.

[0045] The obtained acicular crystals were examined with an X-raydiffractometer, electron probe microanalyzer, and energy dispersivespectrometer. All the acicular crystals consisted of a single crystal ofa Bi-2223 phase and did not contain the element Te that lowers themelting point of host phases.

[0046] The present invention is not limited to the above embodiments,and various modifications may be performed within a scope of the presentinvention. The invention is intended to cover such modifications.

[0047] As described above in detail, according to the present invention,the following advantages can be achieved.

[0048] (A) A method for producing a defect-free acicular crystal havinga Bi-2223 crystal structure is established, thereby obtaininghigh-quality acicular crystals.

[0049] (B) The achievement of the acicular crystal having the Bi-2223crystal structure contributes to the development of superconductingelectronic elements that have been theoretically proposed but have notbeen achieved. Thereby, switching elements operating at high speed andhigh frequencies that have not been used can be achieved and informationtechnology is strongly stimulated.

INDUSTRIAL APPLICABILITY

[0050] The present invention relates to an oxide high-criticaltemperature superconductor acicular crystal that is substantially aperfect crystal and also relates to a method for producing the same.Such a crystal is fit for superconducting devices having a highoperating frequency of several THz.

1. An oxide high-critical temperature superconductor acicular crystalhaving a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, wherein the acicular crystalis grown from a powder compact by heat-treating the powder compact at atemperature of 840 to 890° C. in an atmosphere containing 5 to 100% ofoxygen and has a superconducting critical temperature of 110 K and thepowder compact contains one mol of an oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀crystal structure and 0.2 to 0.8 mol of TeO₂.
 2. A method for producingan oxide high-critical temperature superconductor acicular crystal,comprising a step of heat-treating a powder compact at 840 to 890° C. inan atmosphere containing 5 to 100% of oxygen to grow an acicular crystalhaving a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, wherein the powder compactcontains one mol of an oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure and 0.2 to 0.8 mol of TeO₂.
 3. An oxide high-criticaltemperature superconductor acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀crystal structure, wherein the acicular crystal is grown from a powdercompact by heat-treating the powder compact at a temperature of 840 to890° C. in an atmosphere containing 5 to 100% of oxygen and has asuperconducting critical temperature of 110 K and the powder compactcontains one mol of an oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure, 0.2 to 0.8 mol of TeO₂, and 0.1 to 2.0 mol of CaO.
 4. Amethod for producing an oxide high-critical temperature superconductoracicular crystal, comprising a step of heat-treating a powder compact at840 to 890° C. in an atmosphere containing 5 to 100% of oxygen to growan acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, whereinthe powder compact contains one mol of an oxide having theBi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, 0.2 to 0.8 mol of TeO₂, and 0.1 to2.0 mol of CaO.
 5. An oxide high-critical temperature superconductoracicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, wherein theacicular crystal is grown from a powder compact by heat-treating thepowder compact at a temperature of 840 to 890° C. in an atmospherecontaining 5 to 100% of oxygen and has a superconducting criticaltemperature of 110 K and the powder compact contains one mol of an oxidehaving the Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure and 0.2 to 0.8 mol of anoxide having a (SrCa)₃TeO₆ crystal structure.
 6. A method for producingan oxide high-critical temperature superconductor acicular crystal,comprising a step of heat-treating a powder compact at 840 to 890° C. inan atmosphere containing 5 to 100% of oxygen to grow an acicular crystalhaving a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, wherein the powder compactcontains one mol of an oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystalstructure and 0.2 to 0.8 mol of an oxide having a (SrCa)₃TeO₆ crystalstructure.